This invention relates to a toner for developing an electrostatic latent image and to a device for developing an electrostatic latent image. The present invention is also directed to an image forming method of an oilless fixing type using the above toner.
An electrographic image forming method using a two-component developer composed of a toner and carrier particles is now widely used for obtaining prints and copies.
In recent years, image forming devices are on the increase which have a cleaner for removing residual toner remaining on a photoconductor drum after transfer of a latent image formed thereon with toner and a recycling unit for returning the toner removed by the cleaner to a developing unit, as one disclosed in Japanese Laid-Open Publication No. S60-41079. 
As a toner capable of forming high quality images over a long period of time without deterioration even in a recycling system of a low-temperature fixing type, Japanese Laid-Open Publication No. H07-199538 proposes a toner containing two types of additives, an additive which increases the electrostatic charge amount of the toner and an additive which decreases the electrostatic charge amount of the toner, to improve the fluidity of the toner, and carnauba wax and so on to enhance the releasing property of the toner.
In recent years, many copying machine have a printer function. Such a machine has many opportunities to output only one copy or print, increasing the time during which a developer therein is agitated with respect to the number of copies or prints outputted.
In a developing unit, the agitation of developer gives a large influence to the deterioration thereof. When the developer is drawn up onto a developing roller and the toner and the carrier therein are rubbed with each other in a doctor gap, the temperature of the developer is raised and components of the toner deposit to the peripheral surface of the carrier locally. In an oilless toner, a wax is dispersed to secure fixability and a releasing property of the toner. When heat stress is applied to the developer, too much wax appears on the peripheral surface of the toner and deposits to the peripheral surface of the carrier. In the case where the toner has a negative polarity, when the wax having the same polarity deposits to the carrier, the electrostatic charge amount of the toner is decreased.
As a system for controlling image density, a system which detects the concentration of the toner on a photoconductor with light and controls the amount of the toner contained in the developer according to the detected concentration to control the image density is employed in the image forming apparatuses. When the electrostatic charge amount of the toner is decreased, the system controls to decrease the amount of toner in the developer, so that γ-characteristics can be exhibited up to an intermediate image density region but a saturated image density cannot be obtained.
As a result, there arise problems such as lowering of the image density and insufficient sharpness, and the service life of the developer is considerably shortened.
The problems are typical with high-speed machines in which a developer is passed through a doctor gap of 0.3-1.0 mm so that the thickness of the developer on the developing roller may be constant. The characteristic is dependent on the rotational speed of the developing roller. Normally, the rotational speed of the developing roller is 1.5-3 times that of a photoconductor. In this case, the amount of the developer passing through the doctor gap per 1 second is 5.4-25.2 g per 1 cm of the lateral width of the gap. The above problems occur when the rotational speed of the photoconductor is 240-560 mm/sec, namely, when the amount of developer on the developing roller is 0.15 g/cm2. When the amount of developer passing through the doctor gap is excessively smaller than the lower limit, the stress which the developer receives is too small to cause a problem. When the amount of the developer is excessively larger than the upper limit, the conditions are so bad that it is difficult to solve the problems by improvement of a toner.